Adding per-app default network callbacks support

Adding support for per-app default network changes to send callbacks
used in registerDefaultNetworkCallback() to those applications that
have the default changed as part of the per-app flow.

Bug: 179819249
Bug: 176494815
Test: atest FrameworksNetTests
atest FrameworksNetIntegrationTests
atest CtsNetTestCasesLatestSdk

Change-Id: Id8ef987cdfbaad497929fea21b38f831c4f9b15b